Effects of GTAW Process Parameters On Weld
Effects of GTAW Process Parameters On Weld
Effects of GTAW Process Parameters On Weld
I. INTRODUCTION
Gas Tungsten Arc Welding (GTAW) also known as Tungsten Inert Gas (TIG) welding was developed in late 1930s when a need to
weld magnesium became apparent. The melting temperatures necessary to weld materials in GTAW process is obtained by
maintaining an arc between tungsten alloy electrode and a workpiece. An inert gas sustains the arc and protects the molten metal
from atmospheric contamination. The inert gas may be argon, carbon dioxide, helium or the mixture of these gases.
Advantages & Disadvantages: This method produces high quality low distortion weld, Free of the spatter, welds almost all metals
including dissimilar ones and gives precise control of welding heat.The concentrated nature of a GTAW arc permits pin point
control of heat input to the workpiece resulting in a narrow heat affected zone (HAZ). Narrow HAZ is an advantage because this is
where the base metal has undergone the change due to superheating of arc and fast cooling rate.
Limitations of this process are low deposition rates, requires slightly more welder coordination than Gas Metal Arc Welding or
Shielded Metal Arc Welding [1].
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A. Welding Current:
Welding current represents the flow of electrons. GTAW process is classified as a constant current process as voltage is dynamic in
nature. Welding current selection is very much governed by the tungsten electrode diameter, gas type and welding polarity.
Gharibshahiyanet al. [2] during their study reported that increase in welding current led to the grain refinement in welding metal and
reduces the hardness. This is considered to be attributed to a reduction in the density of dislocations and microstructural coarsening.
Parvinder Singhet al. [3] during their study concluded that increase in current results in increase in deposition rate. It is also
practical that in a given time more heat is needed to melt a given amount of metal. According to joule’s effect, heat is directly
proportional to current and time, given H = I2Rt (where H is heat, I is current, R is resistance, t is time).
Trivedi et al. [4] during their study of weld bead geometry on Aluminium concluded that bead height increases with increase in
current where as bead penetration remains constant with increasing current and Bead width decreases with increase in current.
B. Welding Polarity:
There are three different polarities which might be used when using GTAW depending on the power supply being used. The
direction that the electron flows is referred to as the polarity. Electron generally flows from a negatively charged body to a
positively charged body. If a direct current power supply is used and the workpiece is connected to the positive terminal is called
DCEN. On the other hand if the parent material is connected to the negative terminal of the direct power supply is called DCEP. If
an alternating current power supply is used the polarity is referred to as AC.
Due to the direction of electron movement 70% of the heat in DCEN is directed to the workpiece and 30% of the heat is directed to
the electrode and vice versa when DCEP is used. This results in narrow and deep weld pool in case of DCEN polarity due to high
energy in the parent metal. The arc forces the droplets away from the workpiece due to the low rate of electron emission from the
negative electrode. For a DCEP, weld pool is shallow. This method can be used to clean the surface of the workpiece by knocking
off oxide films by the positive ions of the shielding gas. DCEP produces rapid heating and degradation of electrode tip because
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anode is more heated than cathode. In case of AC polarity 50% of heat is directed towards the workpiece and the other 50% of the
heat is directed to electrode. The AC polarity provides reasonably good penetration of the weld pool and oxide cleaning as shown in
Figure 3[1].
D. Welding Voltage
Voltage controls the length of welding arc and resulting width and volume of arc cone. As voltage increases arc length gets longer
(and arc cone broader), while as it decreases, the arc length gets shorter (and arc cone narrower). A high initial voltage allows for
easy arc initiation and allows for greater range of working tip distance.
Depth of penetration decreases as voltage increases. In GTAW welding process filler feeding or Filler melt off rate should be kept
constant since it is manual process. If not welder has to increase the feed rate of filler as progresses which is tedious and not possible.
Voltage is a controlling variable in manual processes because in manual process it is very difficult to consistently maintain the same
arc length. Hence GTAW is constant current (CC) output method.
Prabhaharan et al.[8] as a result of their study concluded that welding voltage has inverse effect on weld deposit area. Accordingly
increase in welding voltage decreases the deposition rate.
Lakshmansinghet al. during their study on aluminium alloy concluded that increased arc voltage increases the arc length which
results in wider bid width [9].
E. Shielding Gas
Less electrical conductivity of helium than that of argon reduces the diameter of current channel and leads to the current constriction.
Current constriction results in higher peak of heat intensity. Due to higher peak of heat intensity, temperature on anode surface is
approximately twice of that with the use of Argon gas. Heat transportation from electrons is therefore concentrated near arc axis.
CO2 welding provides higher temperature on anode surface than that of Helium gas. Since the molecular weight and hence mole
specific heat of CO2 is higher than He, current constriction near cathode is more in case of CO2 which in turns results in higher
plasma temperature. This phenomenon is explained through modelling by A. Moarrefzadeh[10] as shown in figure.4
Thermal conductivity of Argon shielded arc plasma can be increased by addition of hydrogen in Argon. Lowkeet al.[11] reported
constricted arc plasma due to 10% addition of hydrogen in to argon.
Argon ionization energy is much lower than the He ionization energy due to which ignition can be achieved at higher (up to 13 mm)
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tip to work distance, if combination of He and Ar is used. As the percent of Ar in He increases, spark can be achieved at higher
distance between tip and work [12].
Shanping Lu et al. [12]further concluded that addition of small amount of oxygen (0.2%) in helium argon mixture increases the
depth of penetration and hence depth/width ratio. This is attributed to marangoni convection mode. Generally the surface tension
decreases with the increasing temperature. In the weld pool surface tension is higher in relatively cooler part of the pool edge than
that in the pool center under the arc and hence the fluid flows from the pool centre to the edge. The heat flux is easily transferred to
the edge and the weld shape is relatively wide and narrow. Addition of minor element such as oxygen changes the direction of fluid
flow and Marangoni convection on the pool surface changed from an outward to inward direction which results in deep and narrow
weld shape.
Addition of hydrogen to argon increases the melting efficiency of arc plasma which results in increased depth of penetration, bead
width and reduced hardness of weldment as well as deterioration in mechanical properties [13].
Fig. 4 Distribution of temperature and flow velocity in Argon, Helium and Carbon Dioxide Gas Tungsten Arc at 150AMP arc
current.
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III. CONCLUSIONS
Welding current has an effect on heat input and weld bead geometry. Increasing welding current increases the deposition rate and
bead height at the same time reduces hardness. Increasing the welding current also increase the melting efficiency however the rate
of heat loss to the surrounding is more with increased current.
Welding polarity affects the depth of penetration. DCEN polarity provides deep penetration where as DCEP provides shallow
penetration. Degradation of tungsten electrode is more with DCEP polarity as high is directed towards the electrode with DCEP.
Bead geometry gets affected with weld speed. Depth of penetration increases with increasing welding speed up to the optimum
value then starts decreasing with further increase in welding speed. Bead width decreases with increase in welding speed. Process
efficiency and melting efficiency increases with increase in welding speed.
Depth of penetration and deposition rate decreases with increase in welding voltage. At high voltage arc length increase which
results in wider bid width.
Shielding gas protects the molten metal pool, filler rod, HAZ from air contaminations. Shielding gas affects the arc plasma
characteristics. CO2 gives highest arc constriction as compared to Argon and Helium gas used individually. Ignition characteristics
of argon helps to ignite spark at high work to electrode distance hence argon is mixed in to helium to take advantage of ignition
characteristic. Addition of small amount of oxygen in He-Ar mix, increases the depth of penetration.
Arc temperature near sharp electrode tip is more than that with blunt tip. Arc velocity, current densities and heat flux decreases with
increase in tip angle. Current densities and heat flux at work surface remains unaffected with respect to varying work tip angle.
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